Optics and Lasers

1. B.SC.II
PAPER-B
(OPTICS and LASERS)
Submitted by
Dr. Sarvpreet Kaur
Assistant Professor
PGGCG-11, Chandigarh

2. Unit-IV
Lasers and Fiber
optics

3. LASERS
History of the LASER
• Invented in 1958 by Charles Townes (Nobel prize
in Physics 1964) and Arthur Schawlow of Bell
Laboratories
• Was based on Einstein’s idea of the “particlewave
duality” of light, more than 30 years earlier
• Originally called MASER (m = “microwave”)

4. Laser printer Laser pointer
Laser: everywhere in your life

5. What is Laser?
Light Amplification by Stimulated
Emission of Radiation
• A device produces a coherent beam of
optical radiation by stimulating electronic,
ionic, or molecular transitions to higher
energy levels
• When they return to lower energy levels by
stimulated emission, they emit energy.

6. 6
Properties of Laser
 The light emitted from a laser is monochromatic, that is, it is of one
color/wavelength. In contrast, ordinary white light is a combination of many
colors (or wavelengths) of light.
 Lasers emit light that is highly directional, that is, laser light is emitted as
a relatively narrow beam in a specific direction. Ordinary light, such as
from a light bulb, is emitted in many directions away from the source.
 The light from a laser is said to be coherent, which means that the
wavelengths of the laser light are in phase in space and time. Ordinary
light can be a mixture of many wavelengths.
These three properties of laser light are what can make it more
hazardous than ordinary light. Laser light can deposit a lot of energy
within a small area.

7. Monochromacity
Nearly monochromatic light
Example:
He-Ne Laser
λ0 = 632.5 nm
Δλ = 0.2 nm
Diode Laser
λ0 = 900 nm
Δλ = 10 nm
Comparison of the wavelengths of red and
blue light

8. Directionality
Conventional light source Divergence angle (θd)
Beam divergence: θd= β λ /D
β ~ 1 = f(type of light amplitude distribution, definition of beam diameter)
λ = wavelength
D = beam diameter

9. Coherence
Incoherent light waves Coherent light waves

10. 10
Incandescent vs. Laser Light
1. Many wavelengths
2. Multidirectional
3. Incoherent
1. Monochromatic
2. Directional
3. Coherent

11. Basic concepts for a laser
• Absorption
• Spontaneous Emission
• Stimulated Emission
• Population inversion

12. Absorption
• Energy is absorbed by an atom, the electrons
are excited into vacant energy shells.

13. Spontaneous Emission
• The atom decays from level 2 to level 1 through
the emission of a photon with the energy hv. It is
a completely random process.

14. Stimulated Emission
atoms in an upper energy level can be triggered
or stimulated in phase by an incoming photon of
a specific energy.

15. Stimulated Emission
The stimulated photons have unique properties:
– In phase with the incident photon
– Same wavelength as the incident photon
– Travel in same direction as incident photon

16. Population Inversion
• A state in which a substance has been
energized, or excited to specific energy levels.
• More atoms or molecules are in a higher excited
state.
• The process of producing a population inversion
is called pumping.
• Examples:
→by lamps of appropriate intensity
→by electrical discharge

17. Pumping
•Optical: flashlamps and high-energy light sources
•Electrical: application of a potential difference across
the laser medium
•Semiconductor: movement of electrons in
“junctions,” between “holes”

18. Two level system
absorption Spontaneous
emission
Stimulated
emission
hn hn
hn
E1
E2
E1
E2
hn =E2-E1

19. E1
E2
• n1 - the number of electrons of energy E1
• n2 - the number of electrons of energy E2
•Population inversion-
n2>>n1
2 2 1
1
( )
exp
n E E
n kT
 
 
  
 
Boltzmann’s equation
example: T=3000 K E2-E1=2.0
eV
4
2
1
4.4 10
n
n

 

20. Resonance Cavities
and Longitudinal
Modes
Since the wavelengths involved with lasers and
masers spread over small ranges, and are also
absolutely small, most cavities will achieve
lengthwise resonance
Plane
parallel
resonator
Concentric
resonator
Confocal
resonator
Unstable
resonator
Hemispheric
al resonator
Hemifocal
resonator
c
c
f
f
c: center of curvature, f: focal point
L = nλ

21. Transverse
Modes
TEM00:
I(r) = (2P/πd2)*exp(-2r2/d2)
(d is spot size measured
to the 1/e2 points)
Due to boundary conditions and
quantum mechanical wave
equations

22. Einstein’s coefficients
Probability of stimulated absorption R1-2
R1-2 = r (n) B1-2
Probability of stimulated and spontaneous emission :
R2-1 = r (n) B2-1 + A2-1
assumption: n1 atoms of energy e 1 and n2 atoms of energy e 2 are in thermal
equilibrium at temperature T with the radiation of spectral density r (n):
n1 R1-2 = n2 R2-1 n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)

2 1 2 1
1 1 2
2 2 1
/
=
1
A B
n B
n B
r n  


 

E1
E2

23. B1-2/B2-1 = 1
According to Boltzman statistics:
r (n) = =
1
2 1
2
exp( ) / exp( / )
n
E E kT h kT
n
n
  
1
)
exp(
/
1
2
2
1
1
2
1
2





kT
h
B
B
B
A
n 1
)
/
exp(
/
8 3
3

kT
h
c
h
n
n

3
3
1
2
1
2 8
c
h
B
A n




Planck’s law

24. The probability of spontaneous emission A2-1 /the probability of
stimulated emission B2-1r(n :
1. Visible photons, energy: 1.6eV – 3.1eV.
2. kT at 300K ~ 0.025eV.
3. stimulated emission dominates solely when hn /kT <<1!
(for microwaves: hn <0.0015eV)
The frequency of emission acts to the absorption:
if hn /kT <<1.
1
)
/
exp(
)
(
1
2
1
2 


 kT
h
B
A
n
n
r
1
2
1
2
1
2
1
2
2
1
1
1
2
2
1
2
2 ]
)
(
1
[
)
(
)
(
n
n
n
n
B
A
B
n
B
n
A
n
x 









n
r
n
r
n
r
x~ n2/n1

25. Condition for the laser operation
If n1 > n2
• radiation is mostly absorbed absorbowane
• spontaneous radiation dominates.
• most atoms occupy level E2, weak absorption
• stimulated emission prevails
• light is amplified
if n2 >> n1 - population inversion
Necessary condition:
population inversion
E1
E2

26. How to realize the population inversion?
Thermal excitation:
2
1
exp
n E
n kT

 
  
 
Optically,
electrically.
impossible.
The system has to be „pumped”
E1
E2